The LP noise floor is rather high- maybe -70dB under very good conditions. 12-bit sampling (RMS noise floor of -72dB) would be sufficient for LP use as long as your levels are right (peak signal above -6dB). (12-bit sampling was used for DV but hasn't seen any other widespread use).

The SNR of an MP3 is around 25-30dB. Does this mean that I only need to decode them to 6-bit PCM to capture all the details?

I am curious if anyone has ever done a detailed analysis of an LP's SNR in different frequency bands.

What does "noise" mean in the case of mp3s anyways? How would that noise sound if it wasn't masked? What does a 2.9 bit depth even mean for mp3? How does a codec with ~30 dB signal to noise ratio have a dynamic range of ~150 dB? Honest questions, lossy compression is really starting to confuse me .

And what is the effective SNR of mp3? As in, does an average mp3 sound like a linear PCM file with a SNR of 96? 90? 80? 75? 70?

If mp3 has a signal to noise ratio of only 15-30 dB, how come it is close to the original sound not only when listening but also when looking at it in an audio editor? How come the waveforms are similiar to the original and don't look or sound noisy at all? If it was all psychoacoustic tricks and the waveform was in reality a noisy, 2.9 bit mess, it would show up in an audio editor. Yet there are no obvious faults except for a lowpass for the highest frequencies.